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Publisher Summary This chapter discusses the functional morphology and behavior of shelled veligers and early juveniles. A glance under low microscopic magnification of an actively swimming, straight-hinged veliger, its valves slightly parted, velum fully extended, rapidly beating cilia projecting widely beyond the edge of the glistening valves, brings into view an exquisite, vibrating icon. Clean, translucent, aragonitic valves shimmeringly mirror light as the mite spirals slowly in no particular direction, its velar retractor muscles twitching from time to time in response to a velar collision with some tiny planktonic particle. Not to have seen a living, vibrant molluskan veliger is to have missed one of marine biology's larval treasures. In culture jars, early straight-hinged veliger of Mercenaria mercenaria tends to concentrate in surface layers of seawater. Swarming at the surface could be disadvantageous when upper levels of highly stratified estuaries move seaward more rapidly than the lower layers, movements being accentuated by wind blowing in the same direction.

Publisher Summary Most field predation studies on hard clams, Mercenaria mercenaria , focuses on elucidating information for aquaculture activities and involves manipulative experiments with some form of protection. The control conditions for these studies are used to examine some ecological relationships, but the plots are seeded at relatively high densities. The seed that are placed in the field are much larger than newly settled hard clam recruits. Studies on hard clam larvae and their predators are almost non-existent, and few studies examine predation on newly set individuals. To expand the information available on predators and predation, some predation studies on other bivalves, such as oysters and mussels, and infaunal species, such as cockles, soft-shell, Manila, and bent-nose clams, are included where these species are preyed upon by the same or similar predators as the hard clam. The interactions among sediment type, clam size, predator species, predator size and factors, such as temperature, make all but the most generalized information about consumption rates difficult to interpret.

Summary Mercenaria culture has progressed to where it is a viable and rapidly growing industry, especially in the western mid Atlantic. There are about 300 clam farms operating along the east coast of the United States. Clam culture is carried out in a three step operation — hatchery, nursery, and field grow out. More useful technology is being developed annually which will make the industry more efficient and profitable.

Publisher Summary This chapter discusses the physiological ecology of Mercenaria mercenaria with emphasis on feeding, nutrition, growth, and production. It also includes a review of whole-organism behavior relevant to feeding. A consideration of energy flow through the organism provides the organizing framework. The major terms that deal with energetic are also introduced in the chapter. The various anatomical structures and associated physiological processes are considered in the chapter. The aspects of energy acquisition, which include feeding and various post-ingestion processes, are also reviewed. Energy expenditures such as biodeposition, excretion, and other metabolic “costs” such as respiration are described in the chapter. The vast majority of the research on hard clams is concerned with growth and factors that influence it. The extrinsic (environmental) factors affecting growth are also emphasized. The feeding responses largely explain the patterns of individual growth responses. The chapter also reviews the whole-organism behavior and provides a fluid mechanical perspective on the feeding environment experienced by clams in nature. Feeding occurs close to the bottom, well within the benthic boundary layer, because of the hard clam's infaunal life habit and the relatively short length of its siphons.

Publisher Summary This chapter provides an overview of the history of hard clamming. Hard clams of the genus Mercenaria have been harvested in the Canadian Maritimes and in the United States in all states from Maine to Florida. Native American Indians harvested clams, known in the industry as quahogs from Rhode Island northward and hard clams from Connecticut southward, by treading in wading depths. The Indians ate the meats and used the shells as tools and utensils. The European colonists exploited shallow-water clam populations by treading and using metal rakes with wooden handles. Partly because of legal constraints, the technologies used to harvest clams have since remained as simple rakes, tongs, or dredges, depending on location. As a result, the fishery for clams remains easy to enter because the rakes and boats are inexpensive or can be borrowed from other fisheries and little skill is required for harvesting. Newcomers can simply follow regular fishermen to the beds and harvest nearly as many clams as they do. The main requirements for making good catches are physical strength and determination.

Publisher Summary This chapter discusses the pests, parasites, diseases, and defense mechanisms of the hard clam, Mercenaria mercenaria. Clams, mussels, scallops, and oysters are harvested commercially for many centuries and have suffered from disease-caused mortalities throughout that time. The study of bivalve parasites, diseases, and defense mechanisms is relatively recent, however, driven largely by epizootic mortalities of oysters in the United States and Europe in the past half century. Investigations show that each molluskan group becomes infected by a similar array of organisms from viruses to copepods, although relatively few cause disease. The distinction between infection and disease is important. Infection refers to the establishment of a foreign organism in host tissues. Disease indicates damage to a body part, organ, or system, which may or may not be caused by an infectious agent, such that the affected organism no longer functions normally. In fact, an infection does not necessarily lead to disease. Many infectious agents are parasites that may cause localized tissue damage, but relatively little overall harm to their hosts.

Publisher Summary This chapter discusses the systematics and taxonomy of the hard clam, Mercenaria mercenaria. The specific status of the hard clam is widely accepted, its generic and subfamily placement is still open to controversy among some systematists. A number of common names exist arising from its commercial importance and popularity, and several varieties are described. Paleontological, conchological, anatomical, and biomolecular data indicate that Mercenaria is a major chionine genus, encompassing four subgenera that occur primarily in the northern parts of the Pacific and Atlantic Oceans with most extant species occurring along the coasts of the American continents. Paleontological and conchological data indicate that its ancestors might have originated in the western Pacific and migrated to the Americas during the Lower Oligocene. Mercenaria s.s. radiated into several species in the Miocene, but only three exist today. Of those three, however, Mercenaria mercenaria is the most abundant, and has formed a significant fishery for more than three centuries, as evidenced by the many names given to it by present and past cultures.

Publisher Summary This chapter discusses the management of hard clam stocks, Mercenaria mercenaria . The factor responsible is deterioration of water quality that caused a considerable number of clam grounds to be closed for production. A number of marine animals are important predators on hard clams. If management is successful, the population of predators may increase also, so that predation may increase, and actually take a much heavier toll than before. Reduced hard clam reproductive success will affect the size of the harvest. Changes in water salinity are important. Hard clams require higher salinity than oysters, and storms that open or close access to the ocean can alter salinity and thus affect abundance. Variations in temperature, in water quality, and many other factors can affect reproduction, and cannot easily be identified or controlled. Management must try to be aware, and be prepared to alter adverse effects. Spawner sanctuaries are one way of assuring a regular supply of larvae. The states have taken steps of various kinds to try to deal with the problems. The information required for management of a wild fishery resource must include measures of setting, growth, recruitment, standing stock, and natural and fishing mortality.

Summary It has been 100 years since the publication of the landmark work of Kellog. Although much has been l e a r n e d about the anatomy and histology of Mercenaria mercenaria since that seminal work, we have a long way to travel on that tortuous pathway known as research. A case in point: although we now know much about the cytology, fine understand the life cycle very well and can spawn animals upon demand throughout the year independent of latitude; yet we have no knowledge concerning the fine structure and functions of follicle and nutritive cells of gonadal acini. In the past 15 years, however, substantial strides have been made in renal cytology, fine structure and general function largely due to the work of M.R Morse and her students. It is expected that investigators will direct concentrated studies of this type to all organ systems and cell types. A Mercenaria genome project should not be in the too distant future. A little more than 50 years ago, Thurlow Nelson stated that the eastern oyster, Crassostrea virginica , was the best known marine animal in the world; I expect that with continued and expanded research on thiscomplex, hearty and commercially impo mercenaria may well become the marine animal about which we have the most scientific information.

Publisher Summary This chapter discusses the integrating nutritional physiology and ecology to explain interactions between physics and biology in Mercenaria mercenaria . There are countless numbers of such interactions, even important ones, that enter into the biology of every organism. The chapter also focuses on examination of the important processes where understanding the interactions between physical and biological factors requires integration of nutritional physiology and ecology. The specific interactions that are considered fall into a small number of categories. The complex set of interactions within the seagrass habitat between physical and biological factors that affect the growth of hard clams is reviewed in the chapter. Some important interactions between the physiological state of a hard clam and its ecology, explicitly addressing ecological implications of interactions among multiple physiological stressors are also explored in the chapter. The use of flow chambers in the field and flumes and wave tanks in the laboratory to replace unrealistic observation in still water allows evaluation of the ways in which feeding physiology is influenced by flow regime.

Summary It is apparent that M. mercenaria has been intentionally introduced into the Pacific coast waters of North America in the states of California and Washington, and in England and France, and has probably been transported to England, France, and the West Coast of North America with imports of the American oyster ( Crassostrea virginica ). The source of the Puerto Rico populations is uncertain. It is very likely that more hard clams have been imported into these and other countries than have been recorded. It is also clear that most of these introductions do not lead to established, viable populations, but the exact reasons for success or failure of these introductions are not clear. Examples were noted where reproducing populations were established under appropriate hydrographic and bottom substrate conditions.

Summary Most of the current information on the reproduction of Mercenaria is drawn from field observations and histological examination of collected specimens. In the late 1930s, Dr. Victor L. Loosanoff provided detailed descriptions of spermatogenesis, the juvenile male phase, gametogenesis and spawning in M. mercenaria . Since then a number of researchers have continued to study clams in an attempt to understand the gametogenic cycle, energetic partitioning and fecundity of Mercenaria . However, much remains to be studied with regard to gametogenesis (e.g., a description of oogenesis and the nutritive function of follicle cells, the role selected factors play in timing the gemetogenic cycle, the effect of stressors on gametogenesis, and the specific dietary requirements necessary for gametogenesis), spawning (e.g., spawning cues and the synchrony of events), and reproductive effort (e.g., the development of easier and less time-consuming methods of estimating potential and realized fecundity). With the increased interest in clam aquaculture, significant developments have been achieved with laboratory studies. Dr. Loosanoff and his colleagues at the National Marine Fisheries Laboratory in Milford, Connecticut, again provided the impetus by spawning Mercenaria and other bivave taxa under controlled conditions. Manuals on techniques for conditioning and spawning (e.g., Castagna and Kraeuter, 1981; Hadley et al., 1997 ) are proof that hatchery techniques have advanced to the commercial application stage. An increased understanding of the conditioning requirements and gamete handling procedures (e.g., storage and cryopreservation) would enhance hatchery operations. The marriage of basic and applied research has expanded our knowledge and should provide the direction of future clam research.

Summary The evolutionary history of hard clams has clearly been complex and many aspects remain unclear. The population genetics of hard clams in many ways is unremarkable. These species are wide spread and exhibit relatively little inter-specific divergence, as might be expected in a species that has a high dispersal larval stage. On the other hand, hard clams also exhibit patterns of divergence that indicate that their evolutionary history includes episodes of hybridization that has led to differential patterns of selection and introgression between taxa.

Publisher Summary This chapter discusses the shell structure and age determination of the hard clam, Mercenaria mercenaria. There is an abundance of information on the growth history of individual animals stored within the shells of bivalve mollusks. Analyses of many of these records can provide long time series of individual growth, including periods of time that precede the inception of particular studies. This recording of growth, once deciphered, interpreted, and placed in the context of other time series, can provide valuable data in a wide variety of studies, from marine biological to anthropological. The hard clam, M. mercenaria, by virtue of its widespread distribution, its cultural importance and use for thousands of years, and its shell macro- and microstructure, is well suited for such studies. The shell of the adult hard clam is composed of three aragonitic layers. Proceeding from the shell interior to exterior, these layers are (1) an inner “homogenous,” or complex crossed-lamellar layer; (2) a middle “homogenous,” or complex crossed-lamellar layer; and (3) an outer composite prismatic layer.

Summary It is astonishing that so little information exists on hard clam populations given that this species has supported long-standing, economically important fisheries. A common problem that limits our ability to compare studies and make generalizations about regional or habitat-specific patterns in hard clam population structure is inadequate sampling. Because samplint effort is low (relative to the observed variability), we have little ability to detect differences or to be confident that observed patterns are real. It is tempting to use the data presented here to summarize hard clam populations by speculating on applicable models for their dynamics. Yet the gaps in information, diversity in quality of what is available, and the absence of appropriate between-region comparisons clearly indicate that such an effort is unwarranted. I have even avoided making comparisons to populations of other bivalve species because the limited information on Mercenaria prevents assessing which demographic characteristics can be compared reasonably to other species and which ones cannot. Instead of speculating, I will close by suggesting areas of inquiry that would help us understand hard clam population dynamics better, as revealed by consideration of the data we have available. What is the role of adult dispersion patterns in hard clam spawning success? Does gamete viability, change with age of the adult clam? What are the realized age-specific fecundities of clams?. What are the relative importances of flushing rates and larval predation in determining larval survival? What factors influence settlement success? What proportion of the clams in a new cohort are progeny of the adult clams in the local population? What are the spatial and temporal patterns of abundance of pediveligers and juvenile clams Wherever Mercenaria have been abundant they have been harvested. This places some importance in studying hard clam populations located in areas that have been closed to harvest continuously for several decades. All other existing populations are either at non-equilibrium conditions (due to harvesting) in acceptable hard clam habitats or at equilibrium conditions in marginal hard clam habitats. The data that we gather, from most clam populations will have to be viewed in this context. But the efforts is worthwhile. In the absence of robust information on hard clam populations enlighitened management of this species and understanding their role in estuarine communities will be impossible. Only when unconfounded, long-term abundance data, synoptic life-history information, and experimental studies of factors influencing hard clam demographic parameters have been conduced will it be possible to understand hard clam populations as a comprehensive whole.

Hemocytes of two species of Mercenaria, M. campechiensis and M. mercenaria , were compared relative to size, total cellular protein, and nuclear DNA content by employing flow cytometry. No significant differences were found in these characteristics. Cytometric profiles of nuclear DNA appear superposable for Mercenaria and easily distinguished from Crassostrea, Lymnaea , and the guinea fowl, Numida.

Publisher Summary This chapter discusses the results of experimental investigations on skin digestion in bivalve mollusks and its relation to amino acid absorption from seawater. Recent investigations have demonstrated that several marine invertebrates including bivalve mollusks have a capacity for digestion of material on their body surfaces. It has been suggested that this may provide the epithelial amino acid transport systems with a source of nutrients to supplement those already present and free in seawater. The removal of the dipeptide l -alanyl- l -alanine from seawater by the shipworm Bankia gouldi has been measured using the fluorescamine technique to estimate the level of dipeptide. If the greater inhibition of l -alanine uptake with time has been because of the depletion of energy supplies, then the inhibition by unlabeled l -alanine would also have increased with time. In Guekensia and Crassostrea, the dipeptide may compete directly with amino acids for a common factor in the transport process, although in these animals as in Bankia and Mercenaria, the starch-gel electrophoretic studies of samples collected from their epithelial surfaces demonstrate the presence of several hydrolytic enzymes including leucine-aminopeptidase, apparently in relatively high quantities.

Publisher Summary The determination of the sequence and anomeric linkage of sugar components in the carbohydrate chains of glycoconjugates is difficult because of the limited availability of specific, purified glycosidase. A readily accessible, highly purified preparation of α-L-fucosidase with a broad aglycon-cleaving specificity would provide a valuable analytical tool for structural studies and for the elucidation of the biological functions of fucose-containing glycoconjugates. This chapter discusses the process of α-L-fucosidase purification and properties from Venus Mercenaria. α-L-Fucosidase activity is isolated from the liver (hepatopancreas) of the common edible clam. The enzyme preparation has a pH optimum of 5.5. The standard assay mixture for α-L-fucosidase activity contains 100 μmol of citrate buffer (pH 4.3), 0.1 μmol of p-nitro phenyl α-L-fucopyranoside, and 2–4 μg of enzyme protein in a final volume of 0.15 ml.

Publisher Summary This chapter presents procedure for preparation of Mercenaria paramyosin. Paramyosin prepared from Mercenaria mercenaria can be easily degraded during the extraction procedure, so that great care must be taken to obtain the native protein. Increased yields of native paramyosin can be obtained by suitable modification of the acid extraction procedure. It is suggested that not only 10 mM EDTA, but also 0.5 mM dithiothreitol (DTT), be added to all the solutions used during the extraction. The DTT serves not only to keep the sulfhydryl residues in the reduced state as they are presumed to be in the native molecule, but also to prevent degradation. As a matter of fact, evidence exists that the addition of 0.5 mM DTT to the solutions without any EDTA at all prevents the degradation of paramyosin all by itself.